The invention relates to an exposure controlling apparatus for an electrophotographic copying machine, and more particularly, to such apparatus in which the exposure value, once adjusted by an operator for a particular copying operation, is automatically reset to a normal value upon completion of that copying operation.
One of the factors in obtaining a sharp and clean copy image in an electrophotographic copying machine is the application of a proper exposure to a photosensitive member which has its surface uniformly charged as by corona discharge. An overexposure results in an apparently fair copy free from background smearing (the attachment of toner to non-image areas of the copy) when the electrostatic latent image formed on the surface of the photosensitive member is subsequently developed by a developer which contains toner. However, the density in the image areas diminishes also, with possible consequences that thin lines are not developed in a satisfactory manner or a copy of required legibility is not obtained from an original containing letters and characters of a reduced optical density. Conversely, an underexposure produces a background smearing on the resulting copy. As a result presently available electrophotographic copying machines include some form of sophisticated means which dispenses with the need for close exposure control to assure that a sharp and clean copy is produced from most originals when a copying operation is performed with a given exposure. The use of a developing electrode which is applied with a bias voltage in the developing unit is one example.
However, sophistication in the developing unit cannot assure the reproduction of a sharp and clean copy from every kind of those including originals having an extremely high or low contrast between the image and the non-image area, originals having a blackish or a white background, or the like. It follows therefore that an adjustment of the exposure is still necessary with certain originals. However, it is to be noted that such an exposure control is necessary only with special originals such as those having a colored background, for example, which are infrequently used, and it is desirable that the exposure value be maintained at a predetermined value except when these special originals are used. When an operator produces a copy from a special original by making a corresponding exposure control adjustment and leaves the copying machine under the adjusted condition, another operator may want to use the machine to obtain a copy from an ordinary original. If the second operator operates the machine in the usual manner without noticing the exposure control adjustment made by the preceding operator, a satisfactory copy may not be obtained, since an ordinary original should be copied under the normal or preset exposure condition.
Therefore, it is desirable that an exposure value, once adjusted by an operator for a particular copying operation, be automatically reset to a normal value or a predetermined constant value upon completion of that copying operation.